Where did the UV stretch option go in 2.91?
It's an overlay.
2.8 series introduced the concept of Overlays, which are all the things that are drawn on top of your mesh in the 3D view or the UVs in the UV editor.
From the manual:

The Overlays pop-over configures the overlays that are displayed on
top of images. In the header, there is a button to turn off all
overlays for the UV Editor. This option also toggles the visibility of
UDIMs tile information. The options that are visible in the pop-over
depend on the UV Editor mode.

Stretch falls into this category.